The present invention relates to an ion beam implanter having a plurality of electrostatic quadrupoles for controlling ion beam divergence and, more particularly, to a method of tuning the plurality of electrostatic quadrupoles of such an ion beam implanter.
Ion beam implanters are widely used in the process of doping semiconductor wafers. An ion beam implanter generates an ion beam comprised of desired species of positively charged ions. The ion beam impinges upon an exposed surface of a workpiece such as a semiconductor wafer, substrate or flat panel, positioned in an implantation chamber, thereby xe2x80x9cdopingxe2x80x9d or implanting the workpiece surface with desired ions.
One type of ion beam implanter suitable for deep implantation of ions into a semiconductor wafer workpiece utilizes an radio frequency (RF) accelerator (linac) to accelerate ions to high energy levels on the order of 1 million electron volts (MeV) per charge state. Such an accelerator typically utilizes multiple resonator modules, with each module including an accelerating electrode. The RF accelerator is controlled to take into account the mass, charge and initial velocity of the ions forming the ion beam. After traversing the RF accelerator resonator modules, a focused, high energy ion beam is directed to the workpiece to be implanted. A high energy ion beam implanter having an RF accelerator is disclosed in U.S. Pat. No. 4,667,111, issued on May 19, 1987 to Glavish et al. and assigned to the assignee of the present invention. The ""111 patent is hereby incorporated herein in its entirety by reference.
Both the amplitude (in kilovolts (kV)) and the frequency (in Hertz (Hz)) of the accelerating electrode output signal must be determined as operating parameters for each resonator module. Moreover, when a multiple-stage RF accelerator is utilized, the phase difference ("PHgr") (in degrees (xc2x0)) of each accelerating electrode output signal is a third operating parameter that must be determined. The resonator modules operational parameters of amplitude, frequency and phase must be determined and implemented by the control circuitry and electronics of the ion implanter (in conjunction with a human operator of the ion implanter). This process is referred to as xe2x80x9ctuningxe2x80x9d the ion beam.
A method and system for determining operating parameters of the resonator modules for a multi-stage RF accelerator is disclosed in U.S. Pat. No. 6,242,747, issued on Jun. 5, 2001 to Sugitani et al. and assigned to the assignee of the present invention. The ""747 patent is incorporated herein in its entirety by reference.
In a multi-stage RF accelerator or linac, the ion beam passes through a central opening of the accelerating electrodes of each of the resonator modules. Positioned on either side of an accelerating electrode and axially spaced apart from the accelerating electrode are grounded electrodes. In the two gaps between an accelerating electrode and its flanking grounded electrodes appropriate electrical fields are generated within the gaps by the accelerating electrode to accelerate the ions as they pass through the gaps. For example, as a group of positive ions pass through a gap approaching an accelerating electrode, the accelerating electrode is energized to a negative voltage to generate an axial negative electric field in the gap approaching the accelerating electrode. This negative electrical field causes the positive ions in the particle bunch to accelerate through the negative electric field toward the accelerating electrode.
As the particle bunch of positive ions pass through the accelerating electrode, the voltage of the accelerating electrode is reversed to a positive voltage thereby generating an axial positive electric field in the gap through which the ions travel as they move away from the accelerating electrode. This positive field in the second gap further accelerates the particle bunch. By appropriate choice of module dimension and frequency of electrode energization, alternate ion sources that produce light or heavy ions can be successfully accelerated along the ion beam beam path between an ion source and the implantation chamber so that sufficient energy of the ions is achieved for proper implantation depth of the ions into the workpiece.
One issue that arises in a high energy implanter is that of beam divergence or diffusion. Within each electrode gap, the axial electric field created to accelerate ions within the gap causes radial focusing (that is, narrowing) of the beam in the first half of the gap and radial defocusing (that is, widening) of the beam in the second half of the gap. Unfortunately, because the electric radial defocusing forces in the second half of the gap are stronger than the radial focusing forces in the first half of the gap, the net result is overall radial defocusing as the beam passes through each gap. One method of compensating for radial defocusing is to provide electrostatic lenses, such as electrostatic quadrupoles (xe2x80x9celectrostatic quadrupolesxe2x80x9d), along the beam line to provide for convergence effect on the beam. As many as twelve or more electrostatic quadrupoles may be used along the beam line and may be advantageously positioned within the RF accelerator, in front of the RF accelerator (that is, upstream of the RF accelerator resonator modules), and/or behind the RF accelerator (that is, downstream of the resonator modules).
The basic function of the electrostatic quadrupoles is to focus the beam and to transport the beam from the ion source to the workpiece with a high transmission rate. The transmission rate is defined as the ratio of the final beam current to the injection beam current. The addition of electrostatic quadrupoles, needed for ion beam convergence, complicates the tuning process, because in addition to determining the operating parameters (amplitude, frequency and phase) for the resonator modules, the ion implanter control circuitry (in conjunction with the operator) must also determine operating parameters for the electrostatic quadrupoles. An electrostatic quadrupole is energized by applying a DC voltage to the electrodes of the quadrupole so as to create a DC voltage differential across oppositely positioned electrodes of the quadrupole. Typically, in a unipolar quadrupole there are two electrodes positioned 180 degrees apart, a DC voltage is applied the one electrode while the other electrode is held at ground potential or a reference voltage thereby resulting in an applied DC voltage across the electrode pair. Thus, each quadrupole must be xe2x80x9ctunedxe2x80x9d by determining a magnitude of the DC voltage applied across the quadrupole electrodes such that, in combination with all of the other electrostatic quadrupoles, transmission rate is optimized, that is, the highest transmission rate is achieved while still maintaining suitable beam quality, that is, a suitable beam energy with minimum energy spread. Because of the number of electrostatic quadrupoles in a typical high energy implanter (typically 12), tuning the quadrupoles to achieve a maximum or near maximum transmission rate is problematic.
The resonator modules and electrostatic quadrupoles of present high energy ion beam implanters are typically tuned by an automatic tuning program or software that is part of the ion implanter control electronics. Such an automatic tuning program (xe2x80x9cautotune programxe2x80x9d) utilizes a method of tuning that comprising sequential single parameter tuning, that is, a combination of single parameter tuning steps with each tuning step optimizing or setting a single control variable, that is, determining the amplitude, frequency and phase for each of the resonator modules and determining the magnitude of applied DC voltage for a single electrostatic quadrupole. Using this sequential tuning procedure, the autotune program tunes each parameter, that is, each resonator and each quadrupole individually until a satisfactory or acceptable beam is achieved. An example of a prior art sequential tuning program is depicted in the flow chart of shown in FIG. 3.
Empirical results have shown that the sequential, single parameter tuning of the electrostatic quadrupoles by the autotune program is slow and inefficient. Typically, sequential, single parameter tuning does not find the best beam for implantation, that is, the beam current with the highest transmission rate.
What is needed is an improved method of tuning a plurality of electrostatic quadrupoles of a high energy implanter that is faster than the present sequential, single parameter tuning method and produces a satisfactory beam. What is also needed is an improved method of tuning a plurality of electrostatic quadrupoles of a high energy implanter that generally produces a higher transmission rate tuned beam than the present sequential, single parameter tuning method.
The present invention concerns a method of tuning a plurality of electrostatic quadrupoles. Quadrupoles are used for focusing an ion beam in a high energy ion beam implanter and to transport the ion beam from the ion source injector (where ions are extracted from an ion source) to a workpiece to be implanted with ions which positioned in an implantation chamber. It should be recognized that the method of tuning of the present invention is suitable for use in ion beam implanters whether or not the implanter utilizes an RF accelerator for ion acceleration.
The steps of the electrostatic quadrupole tuning method include: grouping each of the plurality of electrostatic quadrupoles into one of a predetermined number of groups based on a primary function of each quadrupole, the predetermined number of groups being at least one less than a number of electrostatic quadrupoles; and for each of the groups of quadrupoles, tuning the quadrupoles in the group by iteratively substituting values for a voltage to be applied to each of the quadrupoles in the group using a multi-parameter search process and concurrently measuring final beam current measured downstream of the ion accelerator to determine a set of applied voltage values that maximize the final beam current among those applied voltage values tested and utilizing the set of applied voltage values to tune the quadrupoles in the group.
In one preferred embodiment, the predetermined number of groups of electrostatic quadrupoles is three and the primary function of quadrupoles each of the three groups is as follows:
a) group 1xe2x80x94functioning as a matching unit between an analyzing mass unit of the ion beam implanter and the ion accelerator by transforming an emittance orientation of the an ion beam to an orientation of an emittance of the ion accelerator;
b) group 2xe2x80x94transporting the ion beam through the ion accelerator; and
c) group 3xe2x80x94functioning as a matching unit between the ion accelerator and a final energy magnet of the ion implanter by transforming the emittance orientation of the ion beam to an emittance of the final energy magnet.
In this embodiment, the electrostatic quadrupole tuning method is applied, independently on a group by group basis, to the quadrupoles of the each of the three groups and a maximum final beam current is found. If the determined maximum final beam current is found to be suitable, the tuning process is terminated and the quadrupoles are accordingly tuned to achieve the determined maximum beam current (that is, the maximum final beam current found using three group tuning). If, however, the determined final beam is deemed not to be suitable, then the predetermined number of groups is changed from three to one, that is, all of the quadrupoles are combined into a single group and the tuning method of the present invention is applied to the single group including all of the quadrupoles. A new maximum final beam current is found. Generally, this new final beam current will be greater than or equal to the maximum final beam current found through the three group quadrupole tuning process. The quadrupoles are accordingly tuned to achieve the new maximum final beam current.
In one preferred embodiment the invention includes a method of tuning a plurality of electrostatic quadrupole of an ion beam implanter, the steps of the method comprising:
a) grouping each of the plurality of electrostatic quadrupole into one of a predetermined number of groups based on a primary function of the quadrupole, the predetermined number of groups being at least one less than a number of electrostatic quadrupoles; and
b) for each of the groups of quadrupoles, energizing the quadrupoles in the group by iteratively substituting values for a voltage to be applied to each of the quadrupoles in the group using a multi-parameter heuristic algorithm and measuring final beam current measured downstream of the ion accelerator to determine a set of applied voltage values that maximize the final beam current among those applied voltage values tested;
c) measuring one or more parameters of the ion beam upon completion of step (b);
d) determining if the ion beam is acceptable by comparing the one or more measured parameters of the ion beam to one or more standards:
i) if the resulting final beam current is acceptable, then utilizing the determined sets of applied voltage values to energize the quadrupoles in each of the groups; and
ii) if the resulting final beam current is not acceptable, then changing the predetermined number of groups and repeating steps (a)-(d).
As an example, the one or more measured parameters compared to standards could advantageously include final ion beam current, ion beam energy, and ion beam energy spread.
In another aspect of the invention, a method of tuning a plurality of resonators and a plurality of electrostatic quadrupoles of an ion implanter includes the steps of: tuning the plurality of resonators to achieve a desired final beam energy with a minimum energy spread of the ion beam; and tuning the plurality of quadrupoles to maximize a transmission rate of the ion beam where the transmission rate is a ratio of a final beam current of the ion beam measured downstream of the ion accelerator to an injection beam current measured upstream of the ion accelerator.
The same multi-parameter search process used to tune the quadrupoles may also be applied to tune amplitude and phase of the plurality of resonators. Frequency of the resonators is generally set a predetermined value (typically, 13.56 megahertz (MHz)).
The step of tuning of the plurality of quadrupoles including the substeps of: classifying each of the plurality of electrostatic quadrupoles into one of a predetermined number of groups based on a primary function of the quadrupole, the predetermined number of groups being at least one less than a number of electrostatic quadrupoles; and for each of the groups of quadrupoles, tuning the quadrupoles in the group by iteratively substituting values for a voltage to be applied to each of the quadrupoles in the group using a multi-parameter heuristic algorithm and concurrently measuring final beam current to determine a set of applied voltage values that maximize the transmission rate among those applied voltage values tested and utilizing the set of applied voltage values to tune the quadrupoles in the group.
These and other objects, advantages, and features of the exemplary embodiment of the invention are described in detail in conjunction with the accompanying drawings.